The present invention relates generally to wireless communication systems, and more particularly to a distribution network for coupling communication signals between centrally located base station equipment and remotely located subscriber units.
Wireless communication networks, such as cellular mobile telephone and personal communications services (PCS), continue to enjoy wide spread growth and popularity. There is oftentimes a need in such systems to provide increasingly greater call handling capacity, as well as to accommodate higher peak usage. Emerging PCS networks, presently still in the stages of being implemented, demand additional design considerations such as low build out cost as they must compete with entrenched cellular networks.
Several approaches have been adopted for deploying such networks at the lowest possible cost. One approach is to increase the coverage area afforded by a given system by increasing the antenna tower height and transmit power levels beyond conventionally accepted norms. Such solutions, however, often increase the number of xe2x80x9cblindxe2x80x9d spots in areas that include a number of tall buildings, hills, or other natural obstructions to radio propagation.
Alternatively, the operator of the system may deploy a relatively large number of base stations with small footprints. While this avoids blind spots, it greatly increases the total capital cost for base station transceiving equipment which may cost upwards of $200,000 or more per cell site.
Other techniques deploy cells with a relatively small footprint and in a pattern which accommodates the capacity to which the system is eventually expected to grow. Then, rather than deploy base station equipment in each cell (which would be relatively cost prohibitive), backhaul distribution cables, repeaters, translators, or other techniques are used to connect the antennas to centrally located base station equipment.
For example, a suggestion has been made in U.S. Pat. No. 5,381,459 to use cable television networks to distribute wireless communication signals between base station equipment and remote transceiver sites located at each cell. This approach couples the transceiver signals over an existing coaxial cable television network using time or frequency division multiplexing in order to avoid interference with other signals being carried, such as cable television signals.
In addition, coaxial cable television networks suffer from an additional disadvantage of experiencing substantial variations in their electrical properties within even a single network, resulting in varying gain and amplification. Thus, controlling transmission power levels can be difficult. Unfortunately, precise control over transmit power levels is critical to providing reliable data transmission and predictable cell coverage, especially with digital modulation schemes.
The approach described in Patent Cooperation Treaty Application No. WO 97/32442 locates a very high capacity base station near a home cell site. Radio translator equipment units are then located in the outlying cells serviced by the home base station where low traffic density is expected. The translators connect to directional antennas that point back to the home base station site. The translator-to-base-station communication links are then assigned frequencies within the range of frequencies available to the service provider which do not conflict with the frequencies used to carry traffic within the cells themselves.
Even this approach has its shortcomings, however. In particular, initial build out requires the purchase of high capacity, expensive base station equipment which is typically much more expensive than single channel base station equipment. Furthermore, the deployment of such equipment requires complicated frequency reuse engineering as the system capacity increases, so that the frequencies in use within the cells do not conflict with the frequencies in use for backhaul between the cells and the centralized base station equipment.
Therefore, it is typically not possible to justify the cost of deploying complex base station transceiving equipment based only upon the number of initial subscribers. Because only a few cells at high expected traffic demand locations will ultimately justify the expense to build out to higher capacity equipment, the service provider is faced with a dilemma. He can initially build out the system with large footprint cells operating at high power levels and then deploy additional base station equipment by splitting cells and installing new base station equipment and towers as the capacity of the system must be increased. However, this approach not only requires re-engineering of the system every time the capacity increases, but also involves additional logistical difficulties such as obtaining permission to build additional radio towers from local authorities and the like. The other alternative is to use coaxial cable distribution networks, which suffer from unpredictable radiated power levels.
Accordingly, it is an object of the present invention to provide a simple and low cost system for coupling wireless communication signals between centrally located base station equipment and remote cell sites.
It is a further object of this invention to provide such a system with adequate control over signal power levels to enable optimum control of the resulting transmission signal power levels of the remote antennas.
It is a still further object of the present invention to eliminate some of the complexities involved with deploying such systems in the past, such as by reducing the need to mix the various types of physical communication media used to implement such a network.
Yet a further object of the present invention is to provide such a system which may distribute digitally encoded signals, such as Code Division Multiple Access (CDMA) signals, efficiently.
Briefly, the present invention makes use of a broadband distribution network consisting of optical fiber transmission media to distribute signals between centrally located base transceiver station (BTS) equipment and remotely located transceiver equipment referred to herein as xe2x80x9ccable microcell integratorsxe2x80x9d (CMI). With this scenario, a single optical carrier at a given optical wavelength preferably carries the channelized radio frequency signals for a number of different cells or sectors thereof. The CMIs are deployed in a configuration such as one per cell and/or even sectors thereof, to provide radio frequency coverage in a pattern which approximates the eventual expected required deployment of base stations when the system is at full capacity. The same active traffic channels may then be broadcast to multiple CMIs and hence to multiple coverage areas.
Common system elements include a fiber node, which in the forward direction collects the signals radiated by one or more base stations and converts them to suitable format for transmission over the optical fiber such as at a 1550 nanometer (nm) wavelength optical carrier. At the remote sites, an optical fiber xe2x80x9cdropxe2x80x9d or splitter in turn feeds a fiber optic input to each cable microcell integrator (CMI).
The CMIs then convert the information at the optical wavelength to an appropriate radio frequency signal with appropriate forward radiation frequencies. For example, in the PCS bands in the United States, the forward radiated signals may occupy a 1.25 MHz bandwidth at a carrier frequency in the range of 1930-1990 MHz.
Operation in the reverse direction is analogous. The CMIs in each cell or sector receive radio frequency communication signals such as in the PCS band from 1850-1910 MHz and convert such signals to an appropriate optical signal at an appropriate reverse link carrier wavelength such as 1310 nm. The single fiber distribution network then carries the reverse link signal to the fiber node. The fiber node in turn reconverts the optical signals to the appropriate radio frequency signals at the expected PCS reverse link frequencies such as in the band from 1850-1930 MHz.
In a preferred embodiment, the reverse link CMIs generate diversity signals at different RF carriers so that a primary and a diversity channel may be fed back to the BTS.
Multiple CMIs may be configured to transmit and receive at the same RF channel frequencies. A group of CMIs arranged in this manner are referred to as a xe2x80x9csimulcast cluster.xe2x80x9d In comparison to traditional networks wherein the full capacity of an RF channel is not fully utilized, an RF channel may not only be extended, but carefully controlled via the simulcast and is a significant improvement to network efficiency. For example, a group of Up to eight cells may be assigned in a simulcast group thereby increasing service by a single channel transceiver back at the base transceiver system equipment.
Simulcast groups may also be defined by assigning other signal characteristics in common. For example, in CDMA systems, simulcast groups are defined by assigning a common carrier frequency, common pseudonoise (PN) code, and common PN code phase offset.
As a result, a single fiber can be used as the sole distribution physical media to an entire wireless communication network, with the single optical fiber providing an RF carrier to a simulcast group directly off that single fiber.
By providing such simulcast features directly off the single optical fiber distribution network, the need to deploy both optical media, such as fiber optic cable, and radio frequency physical media, such as coaxial cables, is avoided.
Furthermore, radio frequency coverage can be engineered once, at initial deployment, with the CMIs deployed in each sector of each of what will eventually become fully operational cells. As the system is initially brought online, basic coverage is provided by assigning a large number of CMIs as a simulcast group to use the same radio frequency carrier signals. As the need for capacity increases, individual CMIs may be reconfigured to operate on different carrier frequencies, either by manual reinstallation of filtering equipment or by command signals provided over separate control channels who operate at different radio carrier frequencies.
The system thus also avoids the need to redesign the RF coverage characteristics of the system as capacity increases.